This runtime coercion or type test from type 'a to Ellipse

Question

This runtime coercion or type test from type 'a to Ellipse    
involves an indeterminate type based on information prior to this program point.
Runtime type tests are not allowed on some types. Further type annotations are needed.

open System
open System.IO
open System.Net
open System.Net.Sockets
open System.Text
open System.Threading
open System.Collections.Generic
open System.Collections
open Microsoft.FSharp.Collections 
open System.Drawing
open System.Windows.Forms
open System.Collections.Generic
open System.Windows
open System.Windows.Controls
open System.Windows.Markup
open System.Configuration
open System.Data
open System.Data.SqlClient
open System.Configuration
open System.Windows.Media
open System.Windows.Media.Imaging
open System.Collections.ObjectModel
open System.Windows.Shapes
open System.Runtime.Serialization 
open System.Runtime.Serialization.Formatters.Binary
open System.Security.Cryptography
open System.Linq
open System.Reflection
open System.Xml
open System.Xml.Linq
[<AttributeUsage(AttributeTargets.Field, AllowMultiple = false)>]
type UiElementAttribute(name : string) =     
    inherit System.Attribute()   
    new() = new UiElementAttribute(null)    
    member this.Name = name 
    
[<AbstractClass>]type FsUiObject<'T when 'T :> FrameworkElement> (xamlPath) as this =     
    let loadXaml () =        
        use stream = System.IO.File.OpenRead(xamlPath)        
        System.Windows.Markup.XamlReader.Load(stream)    
    let uiObj = loadXaml() :?> 'T         
    let flags = System.Reflection.BindingFlags.Instance ||| System.Reflection.BindingFlags.NonPublic ||| System.Reflection.BindingFlags.Public         
    do         
        let fields =             
            this.GetType().GetFields(flags)             
            |> Seq.choose(fun f ->                 
                let attrs =  f.GetCustomAttributes(typeof<UiElementAttribute>, false)                
                if attrs.Length = 0 then 
                    None                
                else                    
                    let attr = attrs.[0] :?> UiElementAttribute                    
                    Some(f, if String.IsNullOrEmpty(attr.Name) then f.Name else attr.Name)                
            )        
        for field, name in fields do            
            let value = uiObj.FindName(name)            
            if value <> null then                
                field.SetValue(this, value)            
            else                
                failwithf "Ui element %s not found" name     
    member x.UiObject = uiObj 
 
let _random = new Random()
let Damping = 0.95
let Gravity = 0.7;
let Resistance = 0.995
let TotalBalls = 2
(*
        AY : double,
        AX : double,
        X : double,
        Y : double,
        OldX : double,
        OldY : double
*)
type PhysicsInfo() =
    member g.AY with get() = AY and set(value) = AY <- value
    member g.AX with get() = AX and set(value) = AX <- value
    member g.X with get() = X and set(value) = X <- value
    member g.Y with get() = Y and set(value) = Y <- value
    member g.OldX with get() = OldX and set(value) = OldX <- value
    member g.OldY with get() = OldY and set(value) = OldY <- value
    
let InitPhysicsForBall : PhysicsInfo= 
    let p = new PhysicsInfo()
    //_random.NextDouble() * ActualWidth,
    //_random.NextDouble() * ActualHeight,
    p.AY = _random.NextDouble() * 600.0
    p.AX = _random.NextDouble() * 480.0
    p.X = _random.NextDouble() * 10.0
    p.Y =_random.NextDouble() * 10.0
    p.OldX = 0.0,
    p.OldY = 0.0
    //)
    p
let CreateBalls(win : FsUiObject<Window>) = 
    for i = 0 to TotalBalls-1 do 
        let e : Ellipse = new Ellipse()
        e.Width <- 20.0
        e.Height <- 20.0
        
        let _container = win.UiObject.FindName("_container") :?> Canvas
        _container.Children.Add(e)
        let info = InitPhysicsForBall
        e.Tag <- info
let SetConstraints(win : FsUiObject<Window>, ball : Ellipse, info : PhysicsInfo) =
    let _container = win.UiObject.FindName("_container") :?> Canvas
    let bounds : Size = _container.RenderSize
    if info.X <= 0.0 then
        info.X <- 0.0
        info.AX <- info.AX * -1.0 * Damping
    else if info.X >= bounds.Width - ball.Width then
        info.X <- bounds.Width - ball.Width
        info.AX <- info.AX * (-1.0) * Damping
    if info.Y <= 0.0 then
        info.Y <- 0.0
        info.AY <- info.AY*(-1.0) * Damping
    else if info.Y >= (bounds.Height - ball.Height) then
        info.Y <- bounds.Height - ball.Height
        info.AY <- info.AY*(-1.0) * Damping
let UpdateBall(win : FsUiObject<Window>, info : PhysicsInfo) =
    info.AX = info.AX* Resistance
    info.AY = info.AY + Gravity
    info.AY = info.AY * Resistance
    info.X = info.X + info.AX
    info.Y = info.Y + info.AY
    
let CompositionTarget_Rendering(win : FsUiObject<Window>) =
    let _container = win.UiObject.FindName("_container") :?> Canvas
    for i = 0 to TotalBalls-1 do 
        let ball : Ellipse = _container.Children[i] :?> Ellipse  <--- Error
        let info : PhysicsInfo = ball.Tag :?> PhysicsInfo
        UpdateBall(win, info)
        SetConstraints(win, ball, info)
        ball.SetValue(Canvas.LeftProperty, info.X)
        ball.SetValue(Canvas.TopProperty, info.Y)
        
let Window1_Loaded(win : FsUiObject<Window>) = 
    CreateBalls
    CompositionTarget.Rendering += CompositionTarget_Rendering(win);
type MainWindow() as this =
    inherit FsUiObject<Window>(Path.Combine(__SOURCE_DIRECTORY__, "Window.xaml"))
    
[<System.STAThread()>]
do
    let window = new MainWindow()
    window.UiObject.Loaded += Window1_Loaded(window)
    window.UiObject.ShowDialog() |> ignore 

Answer 1

Sorry, it's really hard to parse huge listings. But if compiler cannot infer some type here then the easiest way is to add a type annotation for the type in question

---

Petr

Answer 2

_container.Children[i] :?> Ellipse 

should be

_container.Children.[i] :?> Ellipse 

array indexing in F# needs a dot

---

Brian McNamara [MSFT]